The present invention relates generally to the field of high intensity discharge lamps, and more particularly to the field of arc straightening for such lamps.
HID lamps operated at high frequency are susceptible to acoustic resonances. Standing pressure waves in the lamp can cause the arc to become distorted, to move the arc from side to side, creating an annoying flicker, or even in severe cases to destroy the lamp. One solution to this problem is to operate at a high enough frequency (VHF), such that the acoustic resonances are sufficiently damped to keep the arc stable. Considerations that can impose an upper limit on the VHF frequency include EMI and the switching times of a typical bridge circuit.
It is difficult to guarantee that all lamps designed for a specific ballast will be resonant free at a particular VHF frequency. Two lamps, similarly identical in construction, may have weak instabilities at different frequencies. Lamps of a given wattage may have different chemical fills to provide light with different color temperature. This difference in chemistry may affect the arc stability at VHF frequencies. Differences in how the lamp electrodes function at VHF frequencies may also play a role in the arc stability. One simple approach to avoid weak acoustic resonances at VHF frequencies is to utilize a frequency sweep. However this technique is frequently unsuccessful.
Certain acoustic resonances can be used also in a beneficial way. When a horizontally operated lamp is excited at specific frequencies the arc, which is normally bowed up by convection, becomes straight between the electrodes. This phenomenon is called arc straightening. Frequencies that produce arc straightening are frequently found above the 1st azimuthal acoustic mode of the lamp and below the 1st radial acoustic mode of the lamp. Between these two purely radial acoustic modes are the 2nd azimuthal acoustic mode (purely radial, but relatively weak) and the longitudinal combination acoustic modes associated with these azimuthal acoustic modes. The number of combination acoustic modes depends on the aspect ratio of the lamp. For lamps with low aspect ratio (IL/ID less than xcx9c2) the spacing between the acoustic modes can be sufficient for discrete frequencies to produce arc straightening. This is illustrated in FIG. 1 for a cylindrical 150 W ceramic metal halide lamp with dimensions of 9 mm IDxc3x9713 mm IL (aspect ratio=1.44). FIG. 1 shows the current and power frequencies associated with the acoustic resonances up to the first few combination acoustic modes of the 1st radial acoustic mode. (The numbers under the acoustic modes refer to the longitudinal acoustic mode number for the pure longitudinal acoustic modes and to the longitudinal acoustic mode number of the 1st azimuthal/longitudinal combination acoustic modes. The numbering system is the same for the 2nd azimuthal and 1st radial acoustic modes.) From FIG. 1 one can see that there is a small range or window of frequencies, between resonances, from approximately 15 to 20 kHz current frequency that could produce arc straightening. Note that acoustic resonances are driven by a periodic power input. For sinusoidal type waveforms, the power frequency that excites an acoustic resonance is at twice the current (or voltage) frequency.
For long and thin lamps with higher aspect ratio (IL/ID greater than xcx9c3) the spacing of the combination acoustic modes is much closer and no resonant free windows for arc straightening are apparent. This is illustrated in FIG. 2 for a cylindrical 200 W ceramic metal halide lamp with dimensions of 8 mm ID and 28 mm IL (aspect ratio 3.50). For lamps with higher aspect ratio a frequency sweep over about a 5 or 10 kHz range above the 1st azimuthal acoustic mode and below the 1st radial acoustic mode with a period of about 10 ms can produce arc straightening. For the lamp shown in FIG. 2 the frequency sweep is from about 20 to 25 kHz current frequency.
A second example of a beneficial acoustic resonance comes from excitation of the 2nd longitudinal acoustic mode. By exciting this acoustic mode one can move some of the metal halide chemistry that is segregated near the bottom of a vertically burning lamp higher up into the discharge. This effect can change the color temperature in a vertically burning lamp or increase the lamp efficacy. This effect has been called color mixing. See U.S. Pat. No. 6.184,633.
Utilizing arc straightening with a VHF carrier overcomes potential problems that can occur when a lamp is operated only at VHF. But VHF carrier frequencies may cause weak discharge instabilities, whether caused by acoustic resonances or instabilities at the electrodes. Arc straightening can stabilize the discharge, increasing the range of potential VHF carrier frequencies. The increased freedom to choose a VHF carrier frequency can have advantages in circuit efficiency or in the ability to meet EMI regulations. Especially for long and thin burners operated in horizontal orientation, arc straightening at VHF frequencies can keep the discharge away from the upper wall and prevent cracking of the arc tube.
Briefly, the present invention comprises, in a first embodiment, a method for arc straightening in an HID lamp, comprising the steps of: determining and selecting a frequency signal or a frequency sweep signal that produces arc straightening for an HID lamp; and exciting an arc straightening acoustic mode in conjunction with a carrier frequency signal.
In a further aspect of this embodiment, the step is provided of choosing the carrier frequency signal sufficiently high so that in conjunction with the frequency signal or the frequency sweep signal the arc is stable.
In a further aspect of this embodiment, the exciting step comprises amplitude modulating the carrier frequency signal with either the frequency signal or the frequency sweep signal which corresponds to the power frequencies for arc straightening.
In a further aspect of this embodiment, the step is provided of controlling the amount of arc straightening by controlling an amplitude of the amplitude modulating frequency signal or an amplitude of the modulating frequency sweep signal.
In a further aspect of this embodiment, the exciting step comprises summing the carrier frequency signal with a second frequency signal or second frequency sweep signal to obtain a difference power frequency or power frequencies which excite an arc straightening acoustic mode.
In a further aspect of this embodiment, the step is provided of controlling an amount of arc straightening by controlling an amplitude of the second frequency signal or the second frequency sweep signal relative to the amplitude of the carrier frequency signal.
In a further aspect of this embodiment, the exciting step comprises the step of alternating in time continuously the carrier frequency signal and either a frequency signal or a frequency sweep signal where the frequency signal or frequency sweep signal is equal to one half the power frequency required for producing arc straightening for an HID lamp.
In a further aspect of this embodiment, the step is provided of controlling an amount of arc straightening by controlling a duration of the frequency signal or the frequency sweep signal relative to a duration of the carrier frequency signal.
In a further aspect of this embodiment, the determining step comprises: determining a resonance spectrum for the HID lamp; if a window is present in the resonance spectrum that is above the first azimuthal acoustic mode for the HID lamp and below the first radial acoustic mode for the HID lamp, then selecting a frequency signal that produces arc straightening from within the window; and if the window is not present, then selecting a frequency range for the frequency sweep signal that produces arc straightening that is above the first azimuthal acoustic mode for the HID lamp and below the first radial acoustic mode for the HID lamp.
In a further aspect of this embodiment, the HID lamp has a cylindrical symmetry.
In a further aspect of this embodiment, the HID lamp has a discharge vessel with a ceramic envelope.
In a further aspect of this embodiment, the step of selecting a frequency for producing arc straightening comprises selecting a frequency between a first azimuthal acoustic mode and a first radial acoustic mode in the resonance spectrum for the HID lamp which not only produces arc straightening but also excites the second longitudinal acoustic mode in order to obtain color mixing.
In a further embodiment of the present invention, an HID lamp is provided with arc straightening, comprising: a discharge vessel (3) containing an ionizable filling; and a circuit (300, 302, 304, 306, 308) for exciting an arc straightening acoustic mode in conjunction with a carrier frequency in the discharge vessel.
In a further aspect of this embodiment, the circuit for exciting the discharge vessel includes a component for summing the carrier frequency signal with a second frequency signal or a frequency sweep signal to obtain a difference power frequency signal which excites the arc straightening acoustic mode.
In a further aspect of this embodiment, the circuit for exciting the discharge vessel alternates in time continuously the carrier frequency signal and either a frequency signal or a frequency sweep signal where the frequency signal or frequency sweep signal is equal to one half the power frequency required for producing arc straightening for the HID lamp.
In a further aspect of this embodiment, the circuit for exciting an arc straightening acoustic mode in conjunction with a carrier frequency in the discharge vessel uses a frequency between a first azimuthal acoustic mode and a first radial acoustic mode in the resonance spectrum for the HID lamp which also excites the second longitudinal acoustic mode in order to obtain color mixing.